Such bodies are sometimes called "towed underwater instruments" or, when employed as fishing aids, as "towline depressors". They are used with downriggers in underwater research activity wherein scientific instruments such as seismic recording devices are towed, as well as in deep trolling for fish.
One objective of the invention is to troll or drag an object through the water constantly at a predetermined depth below the surface but above the bottom. In the research fields the object being towed is generally a measuring instrument and in fishing it is a lure or bait.
Traditionally, constant depth trolling or towing has been maintained by towing a sphere or ball which is usually made of lead. Because of its symmetry surface the lead ball provides no net "lifting" effect but is employed because of its extremely high density and its low drag coefficient.
In this specification it should be understood that "lifting" or "lifting effect" really means "sinking" or "sinking effect" , that is, negative lifting or causing the object being towed to stay below the surface.
One of the negatives involved in using a lead ball is its extreme weight. Another negative is the drag of the sphere through the water which reduces its effectiveness at depths where actually it would be desired to tow the instrument or bait. The drag on the ball is a direct function of its speed through the water and the operating depth is also a directed function of speed at which the ball is towed. When not in motion, the ball hangs vertically downward at the full depth of its towline and as towing speed increases the ball moves upwardly into a position directly behind the boat or vehicle which is towing it. The higher the towing speed the higher the ball rides in the water. This is a disadvantage.
This action may be likened to the way a lighter than air balloon acts when tethered by a string. When there is no wind the balloon rises vertically (i.e., at ninety degrees to the ground). Its height is limited only by the length of its string. As the wind begins to blow, the angle of the string becomes less than ninety degrees and the balloon moves closer to the ground, still limited by its string which now is likened to a hypotenuse.
It is an object of this invention to produce a towline depressor which, through its hydrodynamic design will, when towed through water, remain at a substantially constant depth throughout a range of towing speeds.
Another object of this invention is to produce a towline depressor the towing depth of which may be determined solely by the length of the towline through a range of towing speeds.